This type of semiconductor pressure sensor is equipped with a semiconductor substrate whose principle surface corresponds to (110)-face, a pressure detecting diaphragm formed on the principle surface of the semiconductor substrate, and a strain gage resistor constituting a bridge circuit which is formed on the diaphragm and outputs a detection signal in connection with strain of the diaphragm (for example, see JP-A-2001-356061, p3, FIG. 1 (Patent Document 1)).
Here, the strain gage resistor disposed on the (110)-face comprises a pair of center gages arranged at the center portion of the diaphragm along the <110> crystal axis direction, and a pair of side gages arranged to be nearer to the peripheral side of the diaphragm than the center gages (for example, see JP-A-11-94666, p11, FIG. 15 (Patent Document 2)).
Here, FIG. 4 is a diagram showing the arrangement of strain gage resistors Rc1, Rc2, Rs1, Rs2 in a diaphragm 30 formed on the principle surface of a semiconductor substrate 10 of a related art semiconductor pressure sensor using the semiconductor substrate described above. Two crystal axes <110> and <100> which are orthogonal to each other structurally exist on the (110)-face corresponding to the principle surface of the semiconductor substrate 10.
Here, the sensitivity of stress occurring in the <110> crystal axis direction has a remarkably larger piezo-resistance coefficient than the sensitivity of stress occurring in the <100> crystal axis direction, and thus the stress occurring, not in the <100> crystal axis direction, but in the <110> crystal axis direction is used to detect the stress on the (110)-face.
Since <110> of only one direction exists on the (110)-face, the arrangement of the strain gage resistors Rc1, Rc2, Rs1, s2 shown in FIG. 4 must be necessarily adopted to achieve a higher output with respect to the crystal axis having higher sensitivity.
That is, the center gages Rc1, Rc2 disposed at the center side of the diaphragm 30 along the <110> crystal axis direction and the side gages Rs1, Rs2 disposed to be nearer to the peripheral side of the diaphragm 39 than the center gages Rc1, Rc2 are equipped, and the bridge circuit is constructed by these four strain gage resistors to detect the stress occurring in the <110> crystal axis direction.
Specifically, the resistance values of the center gages Rc1 and Rc2 are set to RA and RD respectively, the resistance values of the side gages Rc3 and Rc4 are set to RB and RA respectively, and these strain gage resistors are connected to one another in series to form a rectangular closed circuit, thereby constructing a Wheatstone bridge as shown in FIG. 5.
In the bridge circuit 100 shown in FIG. 5, strain of the diaphragm 30 appears as variations of the resistance values of the strain gage resistors RA, RB, RC, RD under the state that DC constant current I is supplied from an input terminal Ia to an input terminal Ib, and the voltage (detection signal) whose level corresponds to detected pressure, that is, a midpoint potential Vout is output between output terminals Pa and Pb.
Such a semiconductor pressure sensor as described above is normally designed so that a glass seat is attached to the semiconductor substrate 10 by anode bonding or the like as disclosed in the Patent Document 1, for example.
The semiconductor substrate 10 and the glass seat are different in thermal expansion coefficient. Therefore, when the temperature is varied, thermal stress occurs between both the glass seat and the semiconductor substrate 10, and the thermal stress thus occurring is transmitted to the strain gage resistors Rc1, Rc2, Rs1, Rs2 on the diaphragm 30. Here, the thermal stress applied to the center gages Rc1, Rc2 and the thermal stress applied to the side gages Rs1, Rs2 are different from each other in magnitude because the locating positions thereof on the diaphragm 20 are different.
As a result, the difference between the thermal stress applied to the side gages Rs1, Rs2 and the thermal stress applied to the center gages Rc1, Rc2 is output as a noise. Since the difference in thermal stress is nonlinearly varied dependently on the temperature, the temperature characteristic of the offset of the output is curved with respect to the temperature.
Accordingly, some difference occurs between the gradient of the offset with respect to the temperature in the range from the room temperature to a high temperature and the gradient of the offset with respect to the temperature in the range from a low temperature to the room temperature in the temperature characteristic of the offset of the output. This difference is referred to as TNO (Temperature Nonlinearity Offset), and it is an important characteristic to determine the precision of the sensor.
Furthermore, when the semiconductor pressure sensor is promoted to be miniaturized, that is, the semiconductor substrate 10 is promoted to be miniaturized, it is considered to reduce the size of the diaphragm 30 occupying a large area. However, it has been found through studies of the inventors that if the diaphragm 30 is designed in a small size, the difference in thermal stress is increased between the side gages Rs1, Rs2 and the center gages Rc1, Rc2.
Therefore, as the size of the diaphragm 30 is smaller, the TNO characteristic is more degraded. Accordingly, a sensor structure which is improved without degrading the TNO characteristic has been required.